Fast Synthesis of PbS Nanocrystals in Aqueous Solution with Shape Evolution from Cubic to Octahedral Structures and Their Assembled Structures

• Main Authors: Wu, Jian Kuan, 吳建寬
• Other Authors: Huang, Hsuan-Yi
• Format: Others
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/77715040600930700665

碩士 === 國立清華大學 === 化學系 === 100 === Semiconductor nanoparticles are often synthesized in organic solvents with the use of high reaction temperatures. If nanoparticles can be synthesized in aqueous phase, it should reduce the energy cost and the process is more environmentally friendly. According to the literature, PbS nanocrystals are usually synthesized in organic solvents. When they are prepared in aqueous solutions, the reaction mixtures can frequently be heated for 12–24 hours. In this study, we have developed a seeding growth method to synthesize PbS nanocrystals in aqueous solution. The method involves addition of a small volume of preheated lead acetate and thioacetamide (TAA) mixture to an aqueous growth solution of lead acetate, thioacetamide, cetyltrimethylammonium bromide, and nitric acid. By varying the amount of thioacetamide added to the growth solution, PbS nanocrystals with different morphologies were generated in 2 h at 90 ºC. The PbS nanocrystals have sizes of 30–60 nm. Transmission electron microscopy (TEM), powder X-ray diffraction (PXRD) patterns, and scanning electron microscopy (SEM) have been employed to characterize the nanocrystals. Nanocube sizes can also be tuned within a range. UV–vis absorption spectra of PbS cubes, cuboctahedra, and octahedra all show decreasing but continuous absorption from 300 nm to beyond 1000 nm. By monitoring the speed of darkening of solution color, particle growth rate was found to be fastest for nanocubes, followed by truncated cubes, cuboctahedra, and octahedra. The production of different particle morphologies of PbS nanocrystals is linked to their reaction rates. Lowering the concentration of TAA in the reaction mixture can retard the reaction rate, and this favors the formation of octahedra. These monodisperse nanocrystals can readily form self-assembled structures. Truncated cubes and octahedra forming monolayer and multilayer packing arrangements have been studied. PXRD was used to confirm these assembled structures. Intensities of certain peaks in the PXRD patterns are enhanced due to preferred orientations of the nanocrystal packing structures.